1. Field of the Invention
The present invention relates to a power transmission device which is arranged, for instance, between the rear wheel shafts of a 4-wheel drive vehicle.
2. Description of the Prior Art
This type of a conventional power transmission device is disclosed in Japanese Patent Publications No. SHO-58-184322 and SHO-58-77935. This technique is called as the "positive drive unit" in which all force necessary for coupling clutches is transmitted through a cam unit so that great load acts on the cam surface when driving the vehicle. As a result, the cam surface tends to cause stress destruction.
In the conventional power transmission device, oil grooves are formed on the friction faces to pass lubricant quickly so that the friction faces do not slip on each other due to the settlement of lubricant on the friction faces. The conventional power transmission device forms oil films on the friction faces by virtue of the sliding contact between the friction faces in the initial stage of the operation. After the continuous operation of the device, however, lubricant is not successively supplied to the oil grooves, and, in addition, lubricant on the friction faces flows out therefrom due to the centrifugal force that causes the burning of the friction faces.